Since 1990, the wired local area network defined by IEEE (The Institute of Electrical and Electronic Engineers, Inc.) standard 802.3i-1990 has rapidly emerged as the preeminent wired network standard for the office environment, where it has captured an estimated 70% of the market. This network is commonly referred to as 10BASE-T Ethernet, where 10BASE-T is an acronym which stands for 10 Mb/s, baseband transmission, and twisted-pair wiring. In addition to its widespread use in new installations, 10BASE-T Ethernet has in many cases supplanted earlier coaxial cable Ethernet and Token-Ring installations. Its advantages include the use of low-cost twisted pair cable and a star topology which facilitates network change management and fault location. Virtually all computers made today can be configured for Ethernet network access, and the majority of the computers used for business applications require such capability.
Each full-duplex 10BASE-T transceiver transmits data in a packet format to multiple stations on the network through a repeater. A maximum-length packet, which contains a data field of 1500 bytes, occupies the network for a time duration of approximately 1.2 ms. Transceivers are connected to the repeater by a set of transmit and receive twisted pairs. Each transmission is sent on a transmit pair to the repeater, where it is retransmitted on all the receiving pairs except for the originating one. Prior to attempting access to the repeater, each transceiver must determine that the network is idle, a process known as "carrier sensing". However, it is possible for another station on the network to begin transmitting between the time when the network is determined to be idle and the time when a transmission begins. The occurrence of simultaneous transmissions by two or more stations is called a collision, and it is inevitable that some collisions will occur in a carrier-sense, multiple access (CSMA) network such as Ethernet. If the occurrence of a collision can be detected, then colliding transmissions may be terminated at the inception of interference and retransmission can begin immediately. Without collision detection, the transmission medium is unusable for the duration of both damaged packets. Greater network throughput can be achieved by quickly aborting transmissions which are involved in collisions.
When a 10BASE-T network collision occurs, all non-transmitting stations receive garbled data, but each of the colliding stations receives the packet sent from the other transmitter. The occurrence of a collision is readily detected at each transmitting station by observing that a valid data signal is present on the receiving pair during a transmission. Valid data is determined by a combination of amplitude and timing measurements, which is sometimes referred to as a smart squelch. A smart squelch is included in serial network interface controllers, such as the National Semiconductor DP83902A, described in Local Area Networks Databook, 1993 Second Edition, .COPYRGT.National Semiconductor, 1993.
As soon as a collision is detected, the transmitting station terminates the packet with a 32-bit sequence, called "jam", which keeps the collision going long enough to ensure that the transmitting stations will be certain to detect its presence. Non-transmitting stations that have detected the jam sequence can clear their receiver data buffers since the packet is known to be damaged.
After the jam sequence has been transmitted, the stations involved in the collision wait random amounts of time and then attempt another transmission. If another collision occurs, then each station will persist in re-transmitting the data until either a successful transmission occurs or 16 unsuccessful attempts have been made, at which time the packet is discarded and the event is reported as an error. Re-transmissions are separated by a random transmission delay, determined by a truncated binary exponential backoff algorithm, to decrease the likelihood of subsequent collisions.
Prior to 1993, most stations connected to the 10BASE-T network were desktop personal computers; however, it is becoming increasingly common for portable computing units to be equipped with accessory slots conforming to the PCMCIA (Personal Computer Memory Card International Association) or PC Card standards which can accept credit card-sized 10BASE-T adapters. The users of portable computing units would receive significant benefit from a wireless means for extending the 10BASE-T network to establish communication ad hoc among collaborators on a common task, between desktop and portable computers, or with the wired network. Two 10BASE-T stations can form a "network" using a cross-connected cable in which the transmitter of the first station is connected to the receiver of the second station and vice versa. However, since n stations require n/2!.multidot.n-1! duplex twisted-pair cables, it is impracticable for a multi-station ad hoc network to be connected in this manner.
The environment of an ad hoc wireless 10BASE-T network is local, which minimizes the range and coverage requirements, as well as the likelihood of "hidden" stations. Thus, either IR optical or microwave radio frequencies are appropriate to convey the Ethernet signal by baseband on-off keyed modulation. Baseband on-off keyed modulation is spectrally efficient and simple to generate and detect. Signal emission in an IR optical receiver may employ one or more light-emitting diodes, such as the Stanley Electric Co., Ltd. DN304, or laser diodes; while signal emission in a microwave RF transmitter may employ a Gunn diode of the type used in law-enforcement speed detectors. Signal detection in an IR optical receiver may employ one or more photodiodes, while signal detection in a microwave RF receiver may employ a Schottky barrier diode. Information transmitted using IR optical signals is inherently private and resistant to interference since most building materials are opaque. In addition, the generation of IR optical signals does not require a license, and international spectrum regulation issues are avoided.
In the wired 10BASE-T environment, it is certain that the colliding signal originates from a second station. However, if the transmitting and receiving pairs of each station are not connected to the repeater but to a wireless emitter and receiver, respectively, then the wireless receiver will often respond to its own emitter due to reflection from nearby objects or surfaces. Under this condition, the 10BASE-T hardware and protocol will identify every transmission as a collision, the collision protocol will be continuously engaged, and the network will become inoperative. Thus, standard 10BASE-T Ethernet hardware and protocol is inadequate when the wire transmission medium is replaced with a wireless transmission medium.
Collision detection is readily accomplished in prior-art Ethernet systems and star-connected fiber systems determine collisions by means of amplitude detection, since signal attenuation is minimal and all signals have substantially the same amplitude. In 10BASE-2 and 10BASE-5 Ethernet, collisions are sensed by observing a signal level on the coaxial cable which is in excess of that generated by the local transmitter. In 10BASE-T twisted-pair Ethernet and star-connected fiber systems, transmission and reception occur on separate connections, which makes collision sensing even easier to accomplish.
Violations of the coding rules for binary data formats such as Manchester may be detected and used to identify the presence of collisions as described by C. Haegard in U.S. Pat. No. 5,162,791, "Collision Detection Using Code Rule Violations of the Manchester Code", issued Nov. 10, 1992. However, coding rule violations will only occur when the reflected signal from the local transmitter and the colliding signal are commensurate in amplitude. If the colliding signal is strong enough to capture the receiver, which may occur in a collaborative wireless environment, then coding rule violations will not occur.
As known in the prior art, collisions may be sensed by examining the receiver for valid data during a pause in each transmission. If valid data is present, then the transmission is aborted. Since this approach requires modifications to the Ethernet packet structure and the Ethernet transceiver, it has the disadvantage of not being compatible with a standard Ethernet system.
Collisions may also be sensed by a bit-by-bit comparison of a packet being received to the packet being transmitted. Implementation of this method, as described by J. W. Reedy, et al. in "Methods of Collision Detection in Fiber Optic CSMA/CD Networks", IEEE Journal on Selected Areas in Communication, Vol. SAC-3, No. 6, November 1985, results in a more complex MAU (media attachment unit), since data and timing recovery circuits and data buffers must be employed. In fact, these circuits duplicate similar circuits in the attached station.
The prior art in wireless LAN systems includes a variety of IR, or infrared, and RF, or radio frequency, systems. Prior-art IR systems lack the bandwidth, optics or protocol to implement an ad hoc collaborative network of portable computers. Prior art RF systems are either limited by spectrum availability to a data rate less than 10 Mbit/s, or designed for stationary mounting and ac-powered operation.
Since electromagnetic energy is subject to the free-space R.sup.-2 propagation law and may also be scattered, signal amplitude is not constant in wireless systems and the simple, relatively ideal collision detection used in cable or wire systems cannot be employed. Perforce, there are few examples of collision detection which exist in the prior art.
In the prior-art "Vipslan-10/Astrowink-E" infrared wireless Ethernet LAN developed by JVC/NTT Data, the local transmitter signal is cancelled so that a colliding signal may be detected by the Ethernet decoder. However, this approach requires a significant increase in the wireless transceiver complexity, cost, and current drain.
Accordingly, there is a need for a method and device for providing collision detection in wireless carrier sense multiple access systems for ad hoc collaborative networking which is compatible with standard Ethernet protocol and transceivers, and which provides for low complexity, cost, and current drain.